1. Field of the Invention
The present invention relates to the field of four-stroke internal-combustion and direct fuel injection engines. More particularly, the present invention uses intake control in order to reduce the consumption as well as the emissions of such engines.
2. Description of the Prior Art
A high reduction in the cosumption of fuel requires a diluted charge combustion which is generally not compatible with after-treatment of the gaseous effluents.
A large reduction in the consumption of spark-ignition engines requires diluted charge combustion (air and/or EGR) in order to decrease pumping losses of the charge and to increase the polytropic coefficient of the gases, and consequently the efficiency of the four-stroke engine cycle.
However, engines with a highly diluted combustion mixture cannot use an after-treatment of the gaseous effluents by trifunctional catalysis in order to meet the current regulations concerning emission levels, because the fuel/air ratio at the exhaust is below 1 wherein a ratio of 1 represents a stoichiometric mixture with a ratio above 1 being a rich mixture and a ratio below 1 being a lean mixture.
First-generation concepts have already been introduced into the market by automotive manufacturers; these are lean-burn engines (Honda VTE, Toyota Carina, . . . ). They allow urban driving consumption gains of the order of 10% in relation to conventional engines. However, in order to meet emission regulations, these lean-burn engines are generally operated under conventional stoichiometric conditions during cold start-up phases and accelerations. This strategy controls emissions of the vehicle by using a conventional trifunctional catalysis depollution system, but it suppresses any consumption gain of the vehicle during these operating phases.
The second-generation solutions that have followed the first-generation solutions are linked with the development of direct fuel injection. Second-generation systems, coupled with stratified combustion control, allow urban driving consumption gains of the order of 20% by suppressing pumping losses of the four-stroke engine cycle.
Like the first concepts, these engines cannot be subjected to trifunctional catalysis at the risk of nullifying the expected consumption gains.
Generally speaking, the two main problems linked with removing pollution from direct fuel injection engines with stratified combustion are as follows:
Nitrogen oxides emissions cannot be readily subjected to after-treatment by No.sub.x catalysis and they must therefore be low in quantity in the exhaust.
The temperature of the exhaust gases is too low for fast initiation of the oxidation catalyst for conversion of the carbon monoxide and of the unburned hydrocarbons.
Concerning the second problem, heating systems, notably electric systems for heating the oxidation catalyst, have already been considered. However, these systems are costly in energy, which poses problems notably during start-up of the vehicle.